Abstract

The results of an Sm-Nd and Rb-Sr isotope study on the large and rich unconformity-type uranium-(gold-platinum-group elements) deposits in northern Australia indicate identical Sm-Nd ages for primary uranium mineralization at Nabarlek (1,616 + or - 50 m.y.) and Jabiluka II (1,614 + or - 132 m.y.), in excellent agreement with new and previously published Rb-Sr data on U ores and altered host rocks. In contrast, the Sm-Nd ages are older than published U-Pb ages which range from 900 to 1,437 m.y. Sm-Nd data on uraninites from Koongarra are strongly scattered, but the geologic setting and Nd model ages for the least disturbed samples suggest an age between 1,600 to 1,650 m.y., identical to the age of primary mineralization at Nabarlek and Jabiluka II. The narrow range of Sm-Nd ages for these three deposits (1,600-1,650 m.y.) indicates that mineralization took place soon after the deposition of unconformably overlying sandstones and volcanics of the 1,650-m.y.-old Kombolgie Formation.Relatively high initial epsilon Nd values and low 87 Sr/ 86 Sr ratios in the ores indicate that ore fluids derived their metal content from two main sources, a preunconformity source comprising the local wall rocks (lower Proterozoic metasediments and amphibolites) and a postunconformity source including the volcano-sedimentary rocks of the Kombolgie Formation which form part of the evaporite-bearing McArthur Group. At Nabarlek, the Kombolgie Formation isotopic signature is obscured by the intensive wall-rock alteration involving isotopically similar amphibolites and dolerites, but mass balance calculations indicate major contributions of U and heavy rare earth elements from sources outside the alteration zone. The most likely external metal sources are again the postunconformity volcanics and sandstones. In contrast, isotopically more evolved sources such as U-rich late Archean basement gneisses and post-tectonic high-level granites are less probable metal sources on the basis of their much lower epsilon Nd and higher 87 Sr/ 86 Sr values, suggesting that the ore-forming hydrothermal activity did not penetrate the metamorphic basement except in the mineralized shear zones.The new data lend support to models of ore deposition that involve reaction of hot, oxidized, U-bearing formation-meteoric waters, whose source is the postunconformity rocks (e.g., the Kombolgie Formation), with reducing metasediments of the lower Proterozoic basement. Highly altered volcanics within the Kombolgie Formation are favored as major U sources.The origin of the mineralization at Ranger may have involved different processes. Recent U-Pb data indicate a 1,737-m.y. age for mineralization at this deposit, significantly earlier than that of the other deposits and, most importantly, earlier than that of deposition of the Kombolgie Formation. The Sm-Nd data for Ranger uraninites do not form an isochron but indicate a possible age range from 1,600 to [asymp] 1,750 m.y. Data resolution is insufficient to allow reliable distinction between a pre- or post-Kombolgie age. However, Nd-Sr initial ratios for Ranger point to unreasonably high epsilon Nd and high 87 Sr/ 86 Sr value sources when calculated for any age younger than 1,650 m.y., the age of Kombolgie sedimentation and basin formation. In contrast, uraninite Nd-Sr initial ratios at 1,737 m.y. are easily explained as mixtures involving Nd and Sr leached from various lower Proterozoic and late Archean sources. The data are thus more compatible with a pre-Kombolgie origin for uranium mineralization at Ranger.

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